This invention relates to a method of sizing carbon fibers. In recent years, composite materials using carbon fibers are widely utilized in the fields of sports, leisure and aerospace technologies. Such carbon fibers are normally manufactured as filaments or tows and processed into unidirectional sheets, tapes, filament winding, cloths or chopped fibers. In the processing of carbon fibers for unidirectional prepreg widely used in sports, leisure and aerospace technologies, cohesiveness and lubricity are required qualities in order to prevent the occurrence of fluffs and yarn breakage caused by their contact friction with various guide means during yarn-handling processes. In order to obtain unidirectional prepreg sheets of high quality, on the other hand, carbon fiber yarns are required to be able to easily spread thinly and without any gaps. It is an object of the present invention to provide a method of sizing carbon fibers which can satisfy such requirements.
As an example of prior art method of sizing carbon fibers, Japanese Patent Publications Tokko 62-56266 and Tokkai 58-41973 disclosed a method of processing carbon fibers with a water-based emulsion of a sizing composition having epoxy resin derived from bisphenol as main component. Such prior art methods have problems, however, although they are capable of improving cohesiveness of carbon fibers. Firstly, since epoxy resins derived from bisphenol are poor in lubricity, fluffs and yarn breakage occur due to contact friction with various guide means during yarn-handling processes for obtaining a unidirectional prepreg sheet by using carbon fibers sized with such resins. Secondly, since epoxy resins derived from bisphenol have high stickiness, carbon fibers sized therewith have imperfect opening. At a low contact pressure, they cannot be spread into a sufficiently thin sheet and, if they are spread, many gaps are generated as a result. Thirdly, since epoxy resins derived from bisphenol and having a high molecular weight are hydrophobic and highly viscous, sizing compositions having such an epoxy resin as main component cannot easily be made into a stable water-based emulsion with small particle size or be attached to carbon fibers.
Japanese Patent Publication Tokko 62-56267 disclosed another method of sizing carbon fibers according to which carbon fibers are processed by a water-based emulsion of a sizing composition using aliphatic esters as a lubricant in addition to epoxy resins derived from bisphenol. Although lubricity is somewhat improved by this method by the use of aliphatic esters, there still remains the problem of imperfect opening of carbon fibers.
The problem addressed by the present invention, therefore, was that prior art methods could not simultaneously provide sufficient cohesiveness, lubricity and fiber-opening property to carbon fibers and that thin unidirectional prepreg sheets of high quality without any gaps could not be obtained.